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Whether you know it or not, regulatory agencies have long agreed on the presence of and the effects of pesticides with America's drinking water sources. However, understanding the potential effects of chemical mixtures on humans and the environment is one of the most complex problems facing scientists and regulatory agencies. The USEPA has identified this issue as a priority in its research strategy for 2000 and beyond (USEPA, 2000b).

Although guidelines and detailed procedures for evaluating potential effects from exposure to chemical mixtures have been provided by USEPA (USEPA, 1986, 2000b) and other agencies (ATSDR, 2004b), implementation has been difficult because of the complexity of mixtures that occur in the environment and the inadequacy of data on the toxicity of the mixtures. Most toxicological testing is performed on single chemicals-usually at high exposure levels-whereas most human and ecological exposures are to chemical mixtures at relatively low doses. How do humans get exposed to pesticides?

Humans can be exposed to mixtures of pesticides and their degradates that occur in streams and ground water if such water is used as a source of drinking water and if treatment does not eliminate the pesticide compounds. Aquatic organisms are exposed to mixtures that occur in streams. Pesticide mixtures may be derived from common sources (such as point sources) or from multiple nonpoint sources, and may include several different types of pesticide compounds with different mechanisms of toxicity. Although a review of recent research on the effects of pesticide mixtures is beyond the scope of this report, the present approaches taken by USEPA and other agencies for regulating and assessing pesticide mixtures provide an indication of present knowledge and information gaps.

Evaluation and management of potential risks to humans of pesticide mixtures that may occur in drinking water are primarily addressed at the Federal level by USEPA and the Agency for Toxic Substances and Disease Registry (ATSDR). Much of the attention to potential effects of chemical mixtures on human health has been associated with risk assessments required for hazardous waste sites as part of implementing the Comprehensive Environmental Recovery, Compensation, and Liability Act (CERCLA), but specific assessment of pesticide mixtures is also now occurring to meet requirements of the Food Quality Protection Act (FQPA) of 1996.

Under the FQPA, USEPA must assess the cumulative risks of pesticides that share a common mechanism of toxicity, or act the same way in the body. These cumulative assessments consider exposures from food, drinking water, and residential sources. USEPA also incorporates regional exposures from residential and drinking-water sources to account for the considerable variation in potential exposures across the country.

To date, USEPA has determined that within each of four different chemical classes (organophosphates, N-methyl carbamates, triazines, and chloroacetanilides), several specific pesticide compounds have a common mechanism of toxicity and require cumulative risk assessments to better define the potential effects of exposure of humans to multiple pesticides within each class. The potential effects of chemical mixtures on aquatic life have not received as much attention as for human health, although USEPA's Office of Research and Development, National Center for Environmental Assessment, has completed ecological risk-assessment guidelines that support the cumulative risk-assessment approach (USEPA, 2003f).

The pesticide registration and reregistration processes require ecological risk assessment, which includes evaluation by USEPA of the likelihood that exposure to more than one pesticide and its degradates may cause harmful ecological effects. Potential effects of pesticide mixtures on aquatic life also may be considered as part of assessments for National Pollutant Discharge Elimination System (NPDES) permits or hazardous waste sites. Procedures developed by USEPA for conducting assessments for NPDES permits involve a battery of tests, referred to as "whole effluent toxicity" (WET) tests, for both effluents and receiving waters.

With an agressive march toward the protection of source waters from pesticide and chemical mixtures, as well as improving technology to better treat suspect waters, there is hope that the flow of pesticides into humans via drinking water can be brought to a tiny trickle for future generations.